1. Field of the Invention
This invention relates to an electronic oscillators and, more particularly, to an inductor-capacitor (xe2x80x9cLCxe2x80x9d) oscillator or resistor-capacitor (xe2x80x9cRCxe2x80x9d) ring oscillator having a resonant frequency that is controlled by an in-phase voltage.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
Within nearly every electronic subsystem that relies on sequential operations, it is essential to have an oscillator or waveform generator of some sort. The oscillator functions to produce a periodic waveform that is then used to initiate measurements or processes within an electronic subsystem. For example, oscillators are used in any digital or analog electronic circuitry, such as a receiver, transmitter, computer, computer peripheral, and a host of other devices too numerous to mention.
There are many types of oscillators. For example, a simple form of oscillator can be made by charging a capacitor through a resistor (or a current source), then discharging it rapidly when the voltage reaches some threshold, beginning the cycle anew. This form of oscillator is oftentimes known as a relaxation oscillator or RC oscillator. Although simplistic in design, an RC oscillator is sometimes inaccurate or unstable at high frequencies. It is generally understood that crystal oscillators are the most stable and accurate oscillators at high frequencies. Using quartz as a piezoelectric, a crystal oscillator is driven by an applied electric field which, in turn, generates a voltage at the surface of the crystal. The effect is to produce a rapidly changing reactance with frequency. The quartz crystal can, however, be rather expensive and, although stable, oftentimes requires a different quartz crystal each time a different frequency is required.
An LC oscillator is generally regarded to be more accurate and stable at high frequencies then an RC oscillator, yet less expensive to implement and to modulate than crystal oscillators. The tuned LC component of the oscillator is connected to an amplifier which provides gain at the resonant frequency of the LC components. Overall positive feedback is then used to cause a sustained oscillation in the LC components of the oscillator, alternatively known as the xe2x80x9cLC tank circuit.xe2x80x9d As with a crystal oscillator, an LC oscillator can sustain its resonant oscillation using the gain of an amplifier, for example, yet the frequency output of the LC tank can be more readily adjusted.
One mechanism for adjusting the frequency of an LC oscillator is to adjust the capacitor or inductor components. Frequency modulation can, therefore, be performed by changing the inductance or capacitance values in the tank circuit of the oscillator. While voltage-dependent inductors are uncommon, voltage-dependent capacitors are widely used in voltage controlled oscillators (xe2x80x9cVCOsxe2x80x9d). The most common form of a voltage-dependent capacitor is the varactor. While varactors are readily used to modulate the frequency output of an LC or ring oscillator, it is oftentimes difficult to target the exact capacitance value needed to achieve a targeted frequency output. In many instances, parasitic capacitance is associated with the trace conductors attached to each terminal of the capacitor and, therefore, any change to the variable capacitor (varactor) may not account for the parasitic capacitance on the trace conductive lines. The parasitic capacitors add to the total capacitance which, in effect, reduce the percentage variation achievable with the varactor. Laser trimming can be used to further tweek the varactor value, however, laser trimming is costly and impractical as a high throughput manufacturing fix.
It would be desirable to implement an oscillator using, for example, LC components, yet also being able to adjust the frequency output of the oscillator without having to change the capacitive or inductive values of the LC tank circuit. The desired LC oscillator should also be capable of use as a multi-phase oscillator.
The problems outlined above are in large part solved by a circuit and method disclosed herein. The circuit includes an oscillator and, preferably, an LC oscillator or an RC ring oscillator. The oscillator can generate a single-ended output or a differential pair of outputs. If a differential pair of oscillating outputs are generated, the pair of outputs are cyclical waveforms that are a pair of sine waves which are 180xc2x0 out of phase with respect to one another. Yet each of the sine waves are modulated by voltages that are at the same phase angle as the oscillator outputs, or offset by 180xc2x0 from the oscillator outputs.
The oscillator frequency within the circuit are regulated by what is hereinafter known as an xe2x80x9cin-phase voltage.xe2x80x9d The in-phase voltage is of the same phase angle as the oscillating frequency being modulated. If oscillation frequency is to be regulated downward, then the in-phase modulating voltage has a sign of the phase angle that causes the oscillating frequency to decrease. Conversely, if the oscillating frequency is to be increased, then the in-phase modulating voltage has a phase with the opposite sign. Thus, the definition xe2x80x9cin-phasexe2x80x9d refers to either the same phase angle or 180xc2x0 phase angle difference between the modulating voltage waveform and the oscillator output voltage waveform. Using the word xe2x80x9csignxe2x80x9d rather than xe2x80x9cphase,xe2x80x9d it is noted that all phases other than 0xc2x0 or 180xc2x0 are excluded. Importantly, however, in-phase does not refer to a 90xc2x0 out of phase relationship, as in quadrature voltages or quadrature currents. By making the modulated voltage the same phase (i.e., in-phase) as the voltage being placed on the inductor or capacitor of the LC tank circuit, modulation can increase or decrease the oscillating frequency while using less circuitry compared to quadrature current modulation of the frequency. A frequency modulated oscillator can, therefore, be achieved with less complexity yet retaining the stability of LC or RC ring oscillators.
According to one embodiment, a circuit is provided. The circuit includes an oscillator having a capacitor. The circuit also includes a modulator adapted to change a resonant frequency of the oscillator by selectively coupling a modulating voltage upon the oscillator that is in-phase with a voltage placed upon the capacitor. If the resonant frequency is to increase or decrease, the modulating voltage must either by at the same phase angle or offset by 180xc2x0 relative to the oscillating voltage placed on the capacitor.
In addition to the modulator producing an in-phase modulating voltage, the modulator can also produce a modulating voltage which is amplitude regulated. The amount by which the amplitude of the modulating voltage changes will have a direct bearing on the amount by which the oscillator increases or decreases in frequency.
According to yet another embodiment, a multi-phase oscillator is provided. The oscillator includes a plurality of stages coupled in series, where each stage has a corresponding stage whose output is substantially 180xc2x0 out of phase from one of the other stages. In this fashion, multiple stages can be linked together and controlled by a plurality of modulators. The modulators can change the frequency of the oscillator stages by coupling a modulating voltage upon each of the plurality of stages that is in-phase with a voltage placed on an inductor and/or capacitor attributed to that stage. In other words, a modulating voltage is sent to each of the plurality of stages. That modulating voltage has its magnitude and sign modulated by a control voltage.
According to yet another embodiment, a method is provided. The method is used to control a resonant frequency by changing a control signal voltage value to cause a corresponding change in the magnitude and sign of the in phase modulating voltage. The modulating voltage is then inserted in series with an inductor and/or capacitor of an oscillator to change the oscillation frequency of the oscillator dependent on the amount of change in the amplitude and sign (i.e., 0xc2x0 being considered plus and 180xc2x0 being considered minus).